Multiple transducer array



Sept. 29, 1959 v c, s, CLAY, JR 2,906,363

MULTIPLE TRANSDUCER ARRAY I Filed May a, 195; s Sheets-Sheet 1 RECORDER=T0 RECORDER 'b 3 d A? Clarence 8. Clay Jr. Inventor Sept. 29; 1959 c.s. CLAY, JR 7 2,906,363

' MULTIPLE TRANSDUCER ARRAY Filed May 6. 1955 t 5 Sheets-Sheet 3 A 2 lb26 353 (52 d4 5t) 77 C57 66 i 68 -7 Ic 6c c4c .IAI. A .J

FIG-I2 b d' 6) 5 00 00 5 0 C) (5 0 0 0 0 c5 Clarence S. Clay Jr.Inventor BWJ 274 {Attorney Sept. 29, 1959 c. 's. CLAY, JR

MULTIPLE TRANSDUCER ARRAY Filed May 6, 1955 0 4'0 o Io 180 FREQUENCXCYCLES PER SECOND OUTPUT PER GEOPHONE O I I I I l 0 20 4O 6O 80 I00FREQUENCY, CYCLES PER SECOND OUTPUT PER GEOPHONEQ4 0 4'0 ab :20 loFREQUENCX CYCLES PER SECOND o I l o 20 4'0 60 so I00 FREQUENCY, CYCLESPER SECOND OUTPUT PER GEOPHONE Clarence S. Clay, Jr

5 Sheets-Sheet 4 FIG-9 FIG-IO FIGE-II Inventor Sept. 29, 1959 s. CLAY,JR 2,90

MULTIPLE TRANSDUCER ARRAY Filed May 6, 1955 5 Sheets-Sheet 5 so, 6.1 esas 62% FIG-I5 Clarence 8. Clay, Jr. Inventor BYWJJ. 77 Attorney MULTIPLETRANSDUCER Y Clarence S. Clay, Jr., Tulsa, Okla., assignor, by mesneassignments, to Jersey Production Research Company Application May 6,1955, Serial No. 506,615

4 Claims. (Cl. 181-.5)

This invention concerns improvements in the transmitting and receivingof waveform signals, especially with respect to improving thedirectivity of an array of transducers. The invention is of particularutility in the are of seismic prospecting, having application to the useof a pattern of geophones or of shot holes that will furnish a broadband of reduction or rejection of horizontally traveling seismicinterferences.

In the art of seismic prospecting, a seismic disturbance is initiated ata selected point in or on the earths surface, as for example bydetonating an explosive charge in a shot hole, and the resulting waveswhich travel down to the underlying strata and are reflected upward aredetected by geophones at a number of points spread out in a selectedpattern on the earths surface. By determining the arrival time of thewaves at the various detection points and knowing the seismic wavevelocities in the various earth layers, it is possible to estimate thedepths of the several reflecting substrata. Although it is theoreticallypossible to time the arrival of a reflected seismic wave by the use of asingle geophone and recording device, in practice it is usuallydiflicult and sometimes impossible to pick out indicated reflectionwaves from a number of other earth vibrations that are detected andrecorded at the same time. Therefore the usual practice is to employ aplurality of seismometers spread over a considerable distance along theearths surface in a selected pattern as just described and to make aplurality of traces in side-by-side relation on a single chart forpurposes of comparison, since a reflection from a well-defined stratumwill appear on the record as a wave form of increased amplitude on allof the traces in some definite time relation, thus permitting thereflection to be lined up on the record.

It has been found that when difliculty is encountered in obtainingsuitable reflections on the record in some prospecting areas significantimprovements in the ratio of reflection to non-reflection energy or, inother words, in the ratio of essentially vertical-traveling reflectionenergy to essentially horizontal-traveling interference energy can oftenbe obtained by using a plurality of geophones at each detection stationconnected so that their outputs add together, the combined signal beingrecorded as a single trace on the record. This serves to average outsome of the complex earth motions associated with the seismicdisturbance and thus give a simpler record. Nevertheless, such use ofmultiple geophones has left much to be desired.

It is an object of the present invention to provide an arrangement ofgeophones wherein the geophones are so spaced that the response of thearray to horizontally traveling transients will be substantially lessthan that obtained with a conventional array of evenly spaced geophones,thus still further improving the ratio of essentially verticallytraveling reflection energy to essentially horizontally travelinginterfering energy.

In accordance with the present invention an array of geophones is usedcomprising a plurality of sets of uniformly spaced geophones withnon-uniform spacing be tween the sets so that with the overall array thenull wave length characteristics of the sets will be incommensurablewith each other.

Alternatively or in conjunction with such an array of geophones adesired directivity of seismic waves can be obtained by utilizing anarray of seismic shot holes spaced in accordance with the principles ofthis invention.

In a broader aspect of the invention the same princi- 0 ples that applytothe design of such geophone arrays are applicable to arrays of othertransducers such as microphones, acoustic generators and pickups, loudspeakers, electromagnetic wave antennas and the like.

The nature and objectives of the invention are more clearly understoodwhen reference is made to the accompanying drawings in which:

' Figure 1 is a schematic diagram of a portion of a conventionalmultiple geophone spread;

Figure 2 is a schematic plan view of the simplest embodiment of ageophone array in accordance with the present invention utilizing fourgeophones with two geophones in each set;

Figure 3 is a schematic plan view showing an array utilizing twogeophones in each of three sets;

Figure 4 is a schematic plan view of an array utilizing three geophonesin each of two sets;

Figure 5 is a schematic plan view of an embodiment of the inventionutilizing eight geophones in the array;

Figure 6 is a schematic plan view showing an array in which a total ofsixteen geophones is used;

Figure 7 is an exploded presentation of the diagram of Figure 6 showingthe development of the geophone spacing;

' Figures 8 and 9 constitute a graphical analysis of the responsecharacteristics of a geophone array employing evenly spaced geophones;

Figures 10 and 11 constitute a graphical analysis of the responsecharacteristics of an array utilizing sixteen geophones spaced inaccordance with the present invention;

Figure 12 is a schematic plan view of a square areal array oftransducers using the spacing pattern of Figure 5 in each of twoperpendicular directions;

Figure 13 is a similar schematic plan view of a rectangular array oftransducers;

Figure 14 is a schematic elevational view showing a seismic prospectingarrangement in which a plurality of shot points and a plurality ofgeophones are used, with the spacing between shot points beingincommensurable with the spacing between geophones; and

Figure 15 is a similar elevational view showing the equivalence of thearrangement of Figure 14 to one using an incommensurable null spacing ofgeophones.

In Figure 1 a portion of a conventional multiple geophone spread isshown. Usually at least twelve channels will be used in the spread butonly three geophone channels are represented in the figure. Thegeophones 21 are placed upon or embedded in the surface of the ground 20with the separate geophone groups 22. 23, and 24 each positioned at adesired detection station along the profile being prospected. Thegeophones in each group are evenly separated by a distance d. Suitableconnection is made between each of the geophone groups 22, 23, and 24and separate recording channels in the seismic recording apparatus bymeans of separate conductors 25, 26, and 27 in a cable 28.

In accordance with the present invention, as already stated, thegeophones in the array are not spaced evenly. To determine the geophonespacing, consideration is given to the longest wave length ofhorizontally traveling interfering energy that is to be suppressed. Thiswave- Patented Sept. 29, 1959 length is given the designation A. Thelength of each unit of the array is then determined by the relationshipA n l Where "i the num Qt. eop n snherent, l if each set consists of twogeophones thc unit. extends over; /z. wave length. If; three geophones.are; used. in; each set the set will. extend over /3 of a wave lengthand the; phones; w ll e p ced. /3. of ave e th; aP r In the same mannerwith four geophones per unit the set. would, be %.A in length, Thespacing; between sets is. then selected to be incommensurable with thespacing between geophones within the sets. Conyeniently the spacingbetween sets can differ from the spacing between geophones by a functionof a small prime number, for example the half power of 2, 3, 5 7, 1,1,or l3 and so on. The technique, for determining the, wave lengths ofinterfering energy from conventional S eismograrns is well known tothose skilled in the art of seismic prospecting and need not beelaborated upon here.

In Figure 2 an array of four geophones is shown, all connected to thesame conductor a. Geophones 1 and 2 .a1'e wave length apart, geophones 3and 4 are also /2 wave length apart, and geophones, l and 3, are separay the dis nc Alternatively the distance between geophones 1 and 2' couldbe i i Another six-geophone array is depicted in Figure 3 In this. arraythree sets of geophones areused, the spacing between the geophones ineach set and the spacings bebetween the geophones in each set and thespacing: bespaeing between geophones 31 and 32, between geophones 33 and34-, and between geophones 35 and 36. is, in each' instance /2 wavelength, while geophones 31, 33 and 155 are spaced from each otherby 1.2\/ times the wave length. As in Figure :2, all six geophones areconnected to the same conductor to feed a single trace. on the recorder,but the connections are not shown.

In Figure 4 is shown a similar array using three geophones in each set.Geophones 41, 42, and 43 are separated from each other by /3 wave lengthand geophones 44, 45, and 46 are similarly separated from each other by6 wave length. The initial geophones of the two sets, i.e., geophones 41and 44, are separated from each other by the distance in Figure 5 isshown a schematic plan view of a geophone array utilizing eightgeophones which consists of two of the units of Figure 2 with the firstgeophone 5 of the second unit being separated from the first geophone 1of the first unit by the distance N In a similar manner thesixteen-geophone array of Figure 6, as will be seen from the explodedview of Figure 7, consists of the eight-geophone array of Figure 5repeated with the first geophone 9 of the second group of eight spacedfrom the first geophone 1 of the first group of eight by the distance AM1 The separation between geophones in the sixteen-geoa eaee phone arrayis expressed in decimal fractions of a wave length in Figure 6. As inthe previous arrays all of the geophones are connected to a singleconductor leading go the recorder although the connections are not shownere.

Referring now to Figures 8 to 11 inclusive the response characteristicsof a geophone array employing sixteen evenly spaced geophones and theresponse characteristics of an array utilizing sixteen geophones spacedas. in Figure 6. are graphically analyzed and compared.

21: Fi ure 3isr1ib$ ntbd a rap ial ehtesentatibfi. o the assumedamplitude versus frequency spectrum of an interfering transient signalidentified by line SI and the amplitude versus frequency response,curve, identified by line 52 of an array of sixteen evenly spacedgeophones. It will be noted that the geophone array has response nullsat 40, 80, and cycles per second.

Cu e. 5 hows.v he respo se. of e one rr qntinnbi s W v o ea h. par cu fu cy r t e ran e 0t 0 0. out 1 cy e Per I1 Since he p rpos Qt u 5. t q-ma e ma ni e e elati Pha e o e out s snb dn l br gn o the urv a sho n a Pi e m i he lysi use c ns r ctin ure 8 is based an analogy to the ponsePM"? Of an. @l a1fi t d hence curve 51 can be treated as the assumedspectrum? of atransient signal that is to be filtered out. It is moreconvenient to handle the analysis on the basis of fre-. quer eies ratherthan wave lengths although it is to be remembered that it the wavelengths of the interfering waves that are of principal concern. It ismerely n ece? sary to assume uniform velocities of wave propagation tohandle the analysis on a frequency basis.

' By determining the product of the geophone array response curve 52 andthe amplitude of. the transient 51 at each frequency, the curve 53 inFigure 9 is obtained. For comparison line 54 is presented showing theamplitude versus frequency for a desired reflection having a frequencyof :50: cycles. It will be noted fro rn curve 53 that the. interferingsignal has an appreciable amplitude at il 50 W qu y a In Figure 10 ispresented an amplitude versus frequency curve 55 for the geophonearray'of Figure 5. The amplitude versus frequency curve fortheinterfering sigrial is again represented by line 51. Deriving Figure 11from Figure 10 in the same manher that Figure 9 was derived from Figure8, it will be seen that the amplitude versusfrequency curs/e56 is muchimproved over the curve 53 of Figure 9 and that the amplitude of the int erfering signal is greatly reduced in the 50 cycle range.

The same principles of spacing as are illustrated in Figures 2 through 7can also be employed for areal'a'rrays. For example a square arealpattern can be used as shown in Figure 12 in which the spacing patternof Figure 5 is us e d for the separation of the geophones in each row aswell as for the spacing between the rows. Thus geophones 1 and 1b arespaced from each other the saine distance as geophones 1 and 2,geophones 1a and 1c are spaced from each other the same distance asgeophones Sand}? and geophones 1' andla are spaced from each other thesame distance'as geophones 1 and 3;.

Similarly Figure 13 shows a rectangular areal pattern in whichthegeophones 1 through 8 are spaced from each other in the same manneras in Figure '5 and the rows containing the geophones 1a through 8a, lbthrough 85, and 10 through 80 are spaced from the first row in the sameratio as'geophones 1 and 9 in Figure 6 A rectangular array of the typeshown in Figure 13 is also useful .for other types of transducerssuch asloud speakers, other acoustic generators, acoustic detectors, andelectromagnetic wave antennas, when it is desired to produce a narrowbeam of selected directivity. A directional array of this type wouldhave considerable advantage over arrays that depend upon operating thedifferent transdulcers at difierent lev els in order to obtain thedesired directivity. In an array spaced in accordance with Figure 13 orin accordance with similar patterns using the incommensurable nullprinciple, all of the transducers can be identical and can be driven inthe same phase and at the same operating level, thus greatly simplifyingthe setting up of such an array.

Still another modification of the invention is illustrated in Figure 14wherein two or more shot holes 61 and 62 are spaced from each other onthe earths surface 60 a selected distance x and two or more geophones 65and 66 are separated from each other a distance y which isincommensurable with the distance x, the charges in the two shot holesbeing detonated simultaneously and the outputs of the geophones 65 and66 being combined.

Comparison of the diagrams of Figures 14 and 15 will readily reveal thatthe arrangement of Figure 14 is equivalent to an arrangement whereinonly one shot hole 62 is used and four "geophones'are' employed in themanner of Figure 2, geophones 65 and 66 being spaced apart the samedistance y as for the spacing of geophones 63 and 64, and geophones 63and 65 being spaced the distance x from each other. Considering the raypaths from each of the shot holes to a reflecting layer 69 and fromthere to each of the geophones it will be seen that the paths, andtherefore the travel times of the waves, from shot hole 62 to point e togeophone 65 and from shot hole 63 to point f to geophone 66 of Figure 15are of the same length as the ray paths 61 to a to 63 and 61 to c to 64,respectively, of Figure 14. The travel paths 62 to b to 63 and 62 to dto 64 of Figure 14 would of course be the same in Figure 15, but areomitted there to clarify the illustration.

As previously stated, it is also within the contemplation of thisinvention to use a plurality of shot holes spaced in accordance with theincommensurable null principles set forth herein, as Well as such spacedshot holes in conjunction with geophone arrays of the type hereindescribed. In the latter instance the shot hole spacing will be suchthat each null of the shot hole spacing would be incommensurable witheach null of the geophone pattern.

It will be apparent to persons skilled in the art that manymodifications of this invention are possible without departing from itsscope. It is therefore intended that the invention not be limited to thespecific examples presented. The scope of the invention is defined bythe following claims.

What is claimed is:

1. In the art of seismic prospecting an apparatus for detecting seismicsignals which comprises at least two sets of seismic transducers, aplurality of seismic transducers in each set arranged in a line,adjacent transducers in each set being spaced by a selected distance,each set pointing in the same direction, the transducers in each setbeing spaced from the transducers in each other set by a distanceincommensurable with said first selected distance, and means combiningthe outputs of said transducers.

2. An apparatus as defined in claim 1 in which the sets of transducersare arranged along a common line.

3. An apparatus as defined in claim 1 in which the sets are arranged inparallel lines.

4. An apparatus as defined in claim 1 in which the seismic transducersare geophones.

References Cited in the file of this patent UNITED STATES PATENTS2,029,015 Bohm Ian. 28, 1936 2,180,949 Blau et al. Nov. 21, 19392,232,612 Klipsch Feb. 18, 1941 2,232,613 Klipsch Feb. 18, 19412,580,636 Wolf Jan. 1, 1952 2,586,731 Simon et al. Feb. 19, 19522,759,551 Carlisle et al Aug. 21, 1956

